1. Field of the Invention
The present invention relates an image display apparatus that are capable of enhancing visual contrast of a displayed video and reducing power consumption.
2. Related Art
In these years, image display apparatuses typified by liquid crystal displays that have a light source and a light modulation element for modulating light intensity from the light source have become widely available. However, because the light modulation element of such an image display apparatus does not have ideal modulation characteristics, it causes degradation of contrast resulting from leakage of light from the light modulation element especially when black is displayed on the apparatus.
To prevent such degradation of contrast, a number of methods have been proposed for performing luminance modulation of the light source in combination with conversion of the gray-scale level of each pixel of an input video, namely gamma conversion, as appropriate for the input video.
For example, Japanese Patent No. 3215388 describes a technique for determining a backlight luminance and a gray-scale level conversion function (hereinafter a “level conversion function) based on the minimum, maximum, and average gray-scale levels of an input video. JP-A 2005-148710 (Kokai) discloses a technique for generating a histogram of an input video, determining a backlight luminance from the mode, and determining a level conversion function with respect to a bin of the histogram to which the mode belongs.
As compared to an image display apparatus having a constant light source luminance, the above techniques both can enhance contrast by controlling the light source luminance and the level conversion function for an input video as appropriate for the video and also can reduce power consumption because they can lower backlight luminance in accordance with the input video.
However, the technique of Japanese Patent No. 3215388 determines the level conversion function only based on the minimum and maximum gray-scale levels and does not consider the frequency distribution (histogram) of gray-scale levels. It thus has difficulty in obtaining a sufficient contrast for some videos. That is to say, there are a large number of videos that have the same minimum and/or maximum gray-scale level but significantly differ in the distribution of gray-scale levels and the technique sets the same level conversion function for all of such videos, which results in the problem of insufficient contrast of an input video.
The technique of JP-A 2005-148710 (Kokai) determines a level conversion function based on the histogram of an input video and in consideration of the bin to which the mode belongs as well as its frequency. With this technique, however, it is still difficult to obtain a sufficient contrast for a video having a multimodal histogram, such as one having two peaks.